1. Field of the Invention
This invention relates to a brake system for an automotive vehicle and a method of operation thereof and more particularly to a brake system adapted for use on a mine haulage vehicle in which a hydraulically controlled emergency brake is interlocked with the transmission of power to the driven wheels so that application of the emergency brake interrupts the transmission of power to the driven wheels requiring the operator to restart the vehicle before it can be moved.
2. Description of the Prior Art
In underground mining operations, mine haulage vehicles commonly known as "shuttle cars" are employed to transfer loose material dislodged from the mine face rearwardly to a discharge point where the material is transferred onto a conveyor belt that transports the material from the mine. The shuttle car may be electrically driven such as by A.C. or D.C. electric motors or propelled by a diesel engine. A fluid actuated braking system incorporated within a closed hydraulic circuit on the vehicle is operable to apply a braking force to the driven wheels of the vehicle. The braking system is utilized as a service brake on a routine stop, start basis and is actuated in a conventional manner, as by a foot pedal, to energize a master cylinder that pressurizes a conduit to supply fluid to the brakes. The brakes frictionally engage the driven wheels to slow the vehicle or bring the vehicle to a stop.
Operation of the service brakes requires a continuous supply of pressurized fluid. A malfunction of the master cylinder or fluid pumps that generate the required hydraulic pressure may render the service brake inoperable. Therefore, an emergency brake is provided for bringing the vehicle to a stop.
It is the conventional practice to actuate the emergency brake through an operator controllable member, such as a panic bar, positioned in the operator's compartment of the vehicle. Depressing the panic bar releases the hydraulic pressure in the hydraulic circuit that maintains the emergency brakes in a normally deenergized state. Thus, removal of hydraulic pressure from the emergency brake actuates the brake to bring the vehicle to a stop. The practice of actuating the emergency brake through the convenience of a panic bar invites the operator to utilize the emergency brake on a routine basis as a replacement for the service brake. Consequently, the emergency brake is subjected to excessive wear. Such misuse of the emergency brake on a routine basis decreases the overall effectiveness of the emergency brake possibly rendering the emergency brake inoperable in emergency situations.
Another problem encountered with conventional emergency brakes of an electrically powered shuttle car is the loss of brake release pressure or inadequate pressure for release of the emergency brakes which occurs as a result of pump failure or electrical power failure. In the event there is inadequate pressure for brake release or a total loss of pressure for brake release, the emergency brake remains engaged when tramming of the vehicle is resumed. This subjects the emergency brake to undesirable wear. Furthermore, the application of the emergency brake requires excessively high fluid pressure in the control circuit. The operator must raise the fluid pressure in the control circuit or monitor the control circuit for leaks to assure that the pressure does not gradually diminish and result in unintended brake application, which may be partial or total. In many instances it is desirable to maintain the vehicle immovable while performing such operations as unloading the boom and running the conveyor to unload while the vehicle is positioned on an unloading ramp. In such an instance it is essential that the emergency brake be fully engaged to prevent movement of the vehicle while these operations are carried out. To assure that the emergency brake remains engaged when the vehicle is positioned on an incline, a parking lever associated with the emergency brake allows the operator to park the vehicle while other operations are conducted. Disconnecting the main contactors of the electrical controller will also prevent the tramming of the vehicle when positioned on a grade. However, with conventional shuttle cars a solenoid brake operating valve is connected to the main contactors and functions to release the brakes. When electrical power is restored by closing of the contacts within the controller to operate, for example the conveyor, the solenoid valve is energized to release the brakes. This presents a hazardous condition particularly when the vehicle is parked on a grade or ramp.
There is need to provide in a mine haulage vehicle a brake system in which the emergency brake is limited to emergency use only and not as a service brake. The operation of the emergency brake should preclude tramming of the vehicle so that any attempt to move the vehicle would require operational steps in addition to restoring the panic bar to its normal position.